1. Field of the Invention
This invention relates to wings for fitment-constrained air vehicles, and more particularly to wings for tube and pylon launched missiles, projectiles and unmanned aerial vehicles (UAVs) or micro aerial vehicles (MAVs).
2. Description of the Related Art
Air vehicles such as missiles, projectiles and unmanned aerial vehicles (UAVs) or micro aerial vehicles (MAVs) are often launched from ground, air or sea based tube or pylon launch platforms. These vehicles range from a fraction of a pound for MAVs to upwards of 10,000 pounds for large cruise missiles and munitions, and fly with speeds ranging from a few miles per hour to transonic, i.e. around Mach 1. These launch platforms are “fitment” constrained in space and volume e.g. the limited volume of a tube or the limited volume inside or under an airframe. To utilize the available space and volume, these vehicles typically employ retractable wings that are stored inside the airframe and deploy at launch. As used herein a “wing” is any aerodynamic surface that provides flight control and/or lift generation including wings, fins and canards.
Retractable wings are typically formed of machined aluminum. Machined aluminum wings can withstand the heavy loads imparted by transonic flight and/or rapid maneuvering. Aluminum is easily machined to satisfy close tolerances on the “outer mold line” (OML) of the wing. A tight OML tolerance is critical to provide minimal disturbance in aerodynamic performance that could create roll/pitch moments, drag, etc. However, the wings are limited to have a ‘chord’ length “d” less than the diameter of the vehicle and a span length “l” less than the length of the air frame in order to fully retract inside the air frame for storage.
As customer demands on the performance of these types of air vehicles increases and the fitment constraints are restricted further, the limitations on chord and span lengths provide inadequate endurance (range) and control to fly desired missions. By comparison, commercial manned aircraft typically have a chord length that is 3×-4× the diameter of the airframe and a span length of 2× the length of the of the air frame to provide sufficient wing surface area to provide lift at low speeds for efficient flight and maneuverability.
The University of Florida has developed a bendable wing for MAVs (U.S. Pat. No. 7,331,546). The wing may be rolled up around the airframe and the MAV stored in a small cylindrical tube. Upon release, the wing returns to its original position for flight. MAVs are very small, lightweight vehicles that fly at relatively low speeds; hence the loading on the wings is fairly small. The wing is formed from one or more layers of resilient materials such that the wing is bendable from its original position. The resilient materials may include fiber reinforced laminates and fabrics such as carbon fiber reinforced polymers, glass reinforced polymers and aramid reinforced polymers; sheet metal such as spring steel, high strength aluminum, stainless steel and titanium; foam materials; and plastics. The wing returns to its original shape because the elastic characteristics of the wing causes the wing to remain under forces when bent from its original position. These forces abate only when the wing is returned to its original position. The materials used to form the wing have great flexibility and elasticity and bend rather than permanently yielding. Thus the MAV needs only to be removed from a storage container for the wing to return to its original shape. The wing returns to its original shape “without additional steps or use of mechanical mechanisms, such as servos, motors, piezoelectrics, or shape memory alloys.”
Polymers exhibit a glass transition temperature or band of temperatures Tg in which the transition from a glassy state below Tg to an elastomeric state above Tg. Many polymers are formulated for use exclusively in their elastomeric state. If the temperature is reduced below Tg the polymer becomes very brittle and not useful. Other polymers are formulated for use exclusively in their glassy state. If the temperature exceeds Tg, the polymers will decompose and oxidize. An important sub-class of polymers known as “Shape Memory Polymers” (SMPs) are stable (mechanically/chemically/thermally) above and below the glass transition. Virtually any polymer family can be made in an SMP formulation today. The choice as to which polymer is used will depend upon the application. In their elastomeric state the SMPs can be stretched and otherwise deformed. In their glassy state, the SMPs exhibit a high Young's Modulus (at least 10× that in the elastomeric state). The SMP can be deformed from its original shape to some desired shape while in the elastomeric state and then cooled to the glassy state to hold the desired shape. Microscopic strain energy is stored in the molecular strands that provide a small motive force. This sub-class of polymers gets their name from the fact that when the material is reheated to above Tg the SMP returns to its original or “memorized” shape. SMPs have been proposed for use as a deployment mechanism in space-based systems for antennas or solar arrays. In space, the “loads” are minimal because of the lack of atmosphere and gravity. Consequently the motive force of the “memory effect” albeit quite small may be sufficient to deploy certain systems. SMPs have also been proposed for use as a “skin” for reconfigurable wings in aircraft. The skin would be heated to above Tg, the wing reconfigured via actuators and a support structure and then the skin cooled to below Tg to provide a stiff skin. In this case, the motive force of the shape memory effect is negligible.